Description: (Applicant's abstract) Prostate cancer is the most common cancer in American men. These cancer cells initially respond to androgen (A)-withdrawal, undergoing apoptosis and decreased cell proliferation. Later on, they overcome this inhibition and relapse. However, in addition to A-dependent proliferation and a total lack of A response, there is a prostate cancer phenotype whereby proliferation is inhibited by androgens (androgen-induced shutoff). Our research objective is to understand the molecular mechanisms underlying this phenomenon. The AS3 gene was identified as a candidate because its pattern of expression was consistent with that of a mediator of the proliferative shutoff. Functional evidence obtained using expression vectors containing AS3 demonstrated that AS3 triggers a proliferative shutoff. Conversely, antisense AS3 blocks the expression of the A-induced shutoff. The AS3 sequence seems to be a transcription factor with trans-activating protein recognition, and DNA binding domains; it also has a protein kinase motif. Alternatively, AS3 may behave by activating other proteins through phosphorylation, and/or by binding to them. The Specific Aims of this application are designed to elucidate the mechanisms by which AS3 inhibits cell proliferation. Aim #1: Exploring the hypothesis that AS3 is a transcription factor (identification of the downstream mediators of the proliferative shutoff). Aim #2: Investigating the hypothesis that AS3 activates effector proteins by phosphorylating them. Aim #3: Mapping the AS3 pathway. Testing the role of the putative downstream mediators identified in Aim #1 in the shutoff effect. Aim #4: Testing the hypothesis that AS3 expression arrests tumor growth by inoculation of tetracycline-regulated AS3 transfectants into nude mice. We expect that this research will lead to the development of markers to identify this phenotype, and to the development of specific therapeutic strategies.